NARROW LINEWIDTH, DIODE LASER PUMPED, SOLID
STATE LASERS
Nigel R. Gallaher
A Thesis Submitted for the Degree of PhD
at the
University of St Andrews
1994
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Narrow Linewidth, Diode Laser Pumped,
Solid State Lasers
Thesis submitted for the degree of Doctor of Philosophy to the University of St. Andrews
by
Nigel R. Gallaher B.Sc.
J. F. Allen Physics Research Laboratories Department of Physics & Astronomy
University of St. Andrews North Haugh St. Andrews, Fife
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Declaration
î, Nigel R. Gallaher, hereby certify that this thesis has been composed by myself, that it is a record of my own work, and that it has not been accepted in partial or complete fulfilment of any other degree or professional qualification.
Signed Date
I was admitted to the Faculty of Science of the University of St. Andrews under Ordinance General No 12 on 1st October, 1987 and as a candidate for the degree of Ph.D. on 1st October, 1987
Signed Date %
I hereby certify that the candidate has fulfilled the conditions of the Resolution and Regulations appropriate to the Degree of Ph.D.
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ACKNOWLEDGEMENTS
I am indebted to many people for theii* assistance in the completion of this work, namely my reseaich supervisor Professor Malcolm Dunn for his encouragement and patient proof-reading of this thesis. Dr. Bruce Sinclair for his helpful suggestions and infectious enthusiasm for physics, and Dr. Bill Sleat for imparting some of his vast knowledge and experience in the “art” of electronics.
I must also thank Dr. David Tunstall and Dr. Ian Firth for the loan of some essential pieces of apparatus, and Newport Research Corporation for providing the high performance Supercavity miiTors used in this work.
The assistance of the technical support staff in the electronics workshop and mechanical workshop is greatly appreciated with special thanks going to Mr. Jimmy Lindsay for manufacturing the laser cavities and optics mounts so skilfully.
Finally, I owe a gi'eat debt of gratitude to my wife. Dr. Yueping Liu for the support, encouragement and above all friendship, she has provided.
ABSTRACT
The design, consü'uction, evaluation and development of an all solid state, naiTow linewidth laser source is presented. The naiTow linewidth laser system was based on a miniature standing wave NdiYAG laser cavity, end-pumped with lOOmW of 809nm light from a fibre coupled GaAlAs diode laser array. This basic CW laser generated up to 30mW at 1064nm in a single, diffraction limited transverse mode (TEMqo) but multi longitudinal mode output beam. The laser had a pump power threshold of 24mW and an optical to optical slope efficiency of 39%. A simple rate equation based numerical model of this laser was developed to allow various design pai'ameters such as length of Nd:YAG gain medium and amount of output coupling to be optimised. Excellent agreement between the numerical model predictions of the output power as a function of input pump power and experimental data from the optimised multi-longitudinal mode laser was obtained.
To restrict this laser to operate on a single longitudinal mode, twisted cavity mode and intracavity étalon, mode selecting techniques were investigated. Both methods were found to produce reliable single mode laser operation and resulted in output powers at the lOmW level.
the reference cavity transmission and a frequency noise spectr al density of '-20Hz/VHz at IkHz.
In one of the first reported demonstrations of an all solid state injection seeded laser system, this single frequency laser was used to injection seed a diode laser array, transversely pumped, Q-switched Nd:YAG laser to produce 0.25mJ, 35ns pulses in a single longitudinal, single tr*ansverse mode beam.
C
ontents
1
IN T R O D U C T IO N ... 1
1.1 Diode Pumped Solid State Lasers ...1
1.2 Thesis Outline... 7
1.3 References... 9
DIODE LASER PUMPS FOR SOLID STATE LASERS...20
2.1 Introduction... 20
2.2 Single Emitter Diode Laser Design... 20
2.2.1 W avelength Output...23
2.2.2 Maximum Output Power... 23
2.3 High Power Diode Laser Arrays... 24
2.3.1 Diode Laser Arrays and Bars... 24
2.3.2 Two Dimensional Diode Laser Stacks...26
2.3.3 Fibre Coupled Diode Laser Arrays... 27
2.4 Other Areas In High Power Diode Laser Research... 29
2.5 Characterisation of the Pump Diode Laser... 30
2.5.1 Fibre Pigtail Output... 31
2.5.2 Laser Diode Drivers ... 31
2.5.3 Output Power Vs Current Characteristics... 32
2.5.4 Spectral Characteristics... 33
Mode Structure...33
Mode Structure as a Function of Power... 35
Temperature Tuning... 35
CHARACTERISATION AND MODELLING OF THE Nd:YAG
H O L O S T E R IC L A S E R ...44
3.1 Nd:YAG Laser Cavity Design... 44
3.2 Pump Light Delivery System... 45
3.3 Spectroscopic Study of Nd:YAG...47
3.4 Measurement Of Pump Light Transmission Through an NdiYAG Laser Rod... 48
3.5 Optimum Output Coupling...49
3.6 Transverse Mode Quality...53
3.7 Output Spectrum of the NdiYAG Holosteric Laser...54
3.8 Optical to Optical Slope Efficiency...58
3.9 Rate Equation Model of the 1064nm NdiYAG Laser...59
3.9.1 Rate Equation Analysis... 60
3.9.2 Numerical Evaluation of Integrated Single Pass Saturated G ain... 64
3.9.3 R esults...66
3.10 References... 69
C l a p t e 4 LO N G ITU D IN A L M ODE SE LE C T IO N ... 70
4.1 Introduction...70
4.2 Line Broadening and Mode Competition Effects...70
4.2.1 Inhomogeneously Broadened Gain Profiles...71
4.2.2 Homogeneously Broadened Gain Profiles... 72
Spatial Hole Burning...72
4.3 Mode Selection by Manipulation of Laser Parameters ... 76
4.3.1 Short Cavities...76
4.3.2 Laser Operation Close To Threshold... 76
4.4 Interferometric Mode Selectors... 77
4.4.2 Resonant Reflectors...78
4.4.3 Intracavity Tilted Etalon... 79
Losses... 81
Intracavity Etalon Design Criteria,... 82
4.5 Elimination of Spatial Hole Burning...84
4.5.1 Relative Motion of Active Medium and the Laser Cavity...84
4.5.2 Twisted Mode Cavity... 85
4.5.3 Ring Cavity...88
4.6 Experimental Implementation of Etalon Mode Selection...91
4.6.1 Optimum Etalon for the Holosteric Laser ...91
4.6.2 Results from the NdiYAG Laser with Intracavity Etalon Mode Selector... ... 93
4.6.3 Polarisation Splitting...97
4.6.4 Frequency Tuning...99
4.7 The Diode Laser Pumped Twisted Mode Laser... 100
4.7.1 Optical Components... 101
4.7.2 Mechanical Design...102
4.7.3 Characterisation of the Twisted Mode Holosteric Laser...104
A. Output Power and Slope Efficiency...104
B. Spectral Output and Frequency Tuning Performance...105
C. Spatial Hole Burning in the Twisted Mode Laser...107
Birefringence in NdiYAG ... 108
New Twisted Mode Cavity Design...109
Performance of the New Twisted Mode Cavity...I l l 4.8 References... 114
PASSIVE STA BILISA TIO N... 118
5.1 Noise Sources Affecting Laser Frequency... 118
Long-Term D rifts... 119
Short-Term Fluctuations... 120
5.1.2 Fundamental Noise... 121
5.2 Passive Stabilisation of the CW End-Pumped NdiYAG Holosteric Lasers... 123
5.2.1 The Small Anechoic Laser Enclosure... 133
5.2.2 Relative Frequency Stability Between The Etalon And Twisted Mode NdiYAG Lasers in a New Laboratory Environment...138
5.3 Measurement of Frequency Noise Spectral Density... 139
5.4 References... 144
ACTIVE FREQUENCY STABILISATION... 146
6.1 Basic Principals of Active Frequency Stabilisation...146
6.1.1 References For Laser Stabilisation - Frequency Discriminants... 147
Passive Optical Cavities... 147
Spectroscopic Absorption and Emission Lines... 149
6.1.2 Servo Control Electronics... 150
6.1.3 Frequency Control Transducers...152
6.2 Laser Frequency Stabilisation Schemes Based on Passive Optical Reference Cavities... 155
6.2.1 Side-Of-Fringe Locking... 155
6.2.2 Phase Sensitive Detection Locking...157
6.2.3 Limitations of Reference Cavities Used in Transmission Mode...158
6.2.4 Pound-Drever Locking...160
A Phasor Description...164
A Mathematical Description...173
Case 1 - Error signal as a function of optical carrier frequency...175
Case 2 - Frequency Modulation (FM), Ao)n=const...177
6.2.43 Pound-Drever locking and Absolute Frequency
Referencing... 181
6.3 Laser Linewidth Limit Under Servo Control ... 183
6.4 References ... 187
EXPERIMENTAL IMPLEMENTATION OF POUND-DREVER LO CK ING... 194
7.1 Experimental Details... 194
7.1.1 Phase Modulation of the Laser Output... 194
7.1.2 The Optical Isolator... 196
7.1.3 The Ultra-Narrow Linewidth Reference Cavity...196
Cavity Geometry...197
Mode-Matching... 199
Cavity Alignment Procedure... 201
Perf orm ance... 203
7.1.4 Characterisation of the Laser Frequency Transducer...204
7.1.5 The Stabilisation circuitry... 207
7.2 Pound-Drever Locking Servo Loop Performance... 211
7.2.1 Calculation of the Free-Running Frequency Noise Spectral Density 217 7.3 References...221
INJECTION SEEDING AND LOCKING OF HOLOSTERIC L A S E R S... 223
8.1 Introduction...223
8.2 Injection Seeding... 223
8.2.1 Transversely Pumped Q-Switched NdiYAG Holosteric Laser...224
Longitudinal Mode Structure...227
8.3 Injection Locking... 237
8.3.1 Basic Injection Locking Theory... 238
8.3.2 Initial Work on Injection Locking of Two CW Holosteric NdiYAG Lasers... 242
8.4 Frequency Stabilisation By Resonant Optical Feedback... 244
8.5 References...251
C O N C L U S I O N S ... 255
DERIV A TIO N OF POUND-DREVER DISCRIM INANT... 258
R eferences...263
1
INTRODUCTION
1.1 Diode Pumped Solid State Lasers
Many exciting fields of research in optoelectronics, such as coherent optical com m unicationsL2,3,4,5,6^ LIDAR remote sensing"^»*,9,1 0,1 1^ gravitational wave detectorsi2.i3,i4^ laser isotope separation^^d6,i7,i8 and the next generation of optical frequency s t a n d a r d s ^1,22,23,24 based on laser cooled2^»^^»27,28 atoms or ions, rely on lasers with extremely narrow linewidth outputs. Progress in many of these areas has been hindered , however, due to the technical difficulties encountered in producing reliable laser sources having the necessary degree of frequency stability. With the recent revolution in diode laser pumped solid state laser technology, highly efficient and compact all solid-state laser sources are now available which display unprecedented levels of frequency stability. This has stimulated research in the field of ultra-stable lasers and made the development of coherent optical sources a fai* less daunting prospect.
Chapter 1
solid state laser and selective excitation of the TEMqocavity mode. Through temperature tuning effects of the diode laser emission wavelength it is also possible to achieve excellent spectral coincidence between the pump wavelength and a strong absorption band in the solid state gain medium. This results in optical-optical conversion efficiency in the holosteric laser as high as 30-40% and overall electrical to optical or “wall plug” efficiency approaching 20%. In comparison, a good flash pumped systems will have a wall plug efficiency in the region of only 5%.
Such high conversion efficiencies in diode pumped lasers means that waste heat production in the solid state gain medium is minimised and so forced air or water cooling of the gain medium is often unnecessary. As well as simplifying laser design, the absence of any forced cooling of the laser greatly reduces the technical noise (mechanical vibrations and acoustic noise) in the laser head resulting in excellent free running frequency stability. Diode pumped solid state laser systems can in general be made extremely compact and robust and so can be easily isolated from the environment, further improving their free running stability performance. For instance, diode pumped solid state lasers have been demonstrated with free running, short term linewidths in the region of a few kilohertz29,30,31,32,33 w ith the application of the advanced frequency stabilisation technique of Pound-Drever locking^"^, this already naiTow linewidth can be dramatically reduced to the millihertz leveP5.36,37^ In contiast, conventional flash lamp pumped solid state lasers have intiinsic linewidths typically in the region of 30-40 MHz. Even under active frequency control, these lasers have only achieved linewidth down to 100-200 kHz38,39_
Cfm pîerl
through Q-switching techniques. High output powers in diffraction-limited beams opens up many opportunities in efficient nonlinear frequency conversion processes such as frequency doubling^o,41,42,43^ sum frequency mixing^ and more recently, efficient and widely tunable all solid state optical parametric oscillators This and the wide variety of other solid state gain materials, which can be pumped by diode lasers, provide access to wavelength regions difficult to reach with today’s diode lasers.
This holosteric approach to laser design was in fact proposed back in 1963 by Newman"^^ shortly after the invention of the semiconductor diode laser. Even at this early stage in the history of laser development, many of the advantages of an all solid state laser system were already apparent. The first successful demonstration of a diode laser pumped solid state laser was performed by Keyes and Quist'^^ in 1964, using a GaAs diode laser to optically pump U:Cap2 operating at 4K. Despite this early demonstration of the diode pumped solid state laser principle and other subsequent demonstrations over the course of the late 1960s^®»^h52 ^9 7 0 3 5 3,54,55,55 progress was slow and experiments achieved httle more than proof of principle status. Realisation of tlie fuU potential of diode pumped solid state lasers was hampered at this stage by the lack of development in the newly emerging field of semiconductor diode lasers. These early pump sources tended to be unreliable and the optical output power they could deliver was limited to only a few milliwatts. Another limitation to the practicality of these early diode pumped solid state lasers was the need for cryogenic cooling of the semiconductor diode lasers and frequently of the solid state gain medium also, in order to reduce laser threshold and to achieve spectral overlap of the diode laser emission and the absorption bands of the solid state gain medium through temperature tuning effects.
Chapter 1
longitudinally pumped by a single stripe GaAs/GaAlAs laser delivering 10-40mW of optical power depending on the particular make of the diode laser used. Although the output power from the diode pumped solid state laser was comparatively modest at around a few milliwatts, this simple laser displayed an excellent optical-optical slope efficiency of 25% and unprecedented levels of overall electrical-optical efficiency and free running frequency stability of 6.5% and sub-lOkHz over 0.3sec respectively.
Chapter 1
the field of single frequency holosteric lasers a particularly noteworthy and highly successful laser design is the monolithic NonPlanar Ring Oscillator (NPRO) invented by Kane et al^^ and subsequently modified by Trutna et al^"^. In this unidirectional ring laser all the elements of the intracavity optical isolator are embodied in the monolithic gain medium itself. The ring path is defined by four reflectors, three of which are provided by total internal reflection within the gain medium. A concave multilayer dielectric mirror used at oblique incidence acts as a partial polariser as well as the fourth cavity reflector. A magnetic field applied to the Faraday active gain medium forms the nonreciprocal (direction dependent) rotator whilst the reciprocal (direction independent) rotation is generated by the out-of-plane total internal reflections in the crystal^^. The dimensions of monolithic device are typically 5x4x2mm. The NPRO laser has proved ideal for diode laser pumping, capable of high output power (0.9 IW)^^ and excellent free running frequency stability (3-10kHz)^?78 ^nd has become a successful commercial product^^. Fast piezoelectric frequency tuning has also been demonstrated in monolithic^® and two- piece*^ diode pumped NPRO devices as well as NPRO operation at other wavelengths (1.319 and 1.338|im)24 and in other materials such as neodymium doped gadolinium gallium garnet (NdiGGG)*^
Chapter I
commercial cutting and polishing techniques and allows the fabrication of small rods, cubes, rhombs and other more exotic geometry used in diode pumped laser systems.
Finally, the thermal conductivity of NdiYAG is high. Whilst this feature is perhaps of little concern in low power laser diode pumped systems because of the small thermal load on the laser rod, it is an important consideration in high power systems where thermal lensing and even fracturing of the laser rods are major problems*"^. A more comprehensive discussion of the mechanical, optical and lasing properties of NdiYAG can be found in, for example, Koechner*^ and Zverev et al*®.
( F5/2+ «=5
'3/2 »«::in
4ihs/2
4th3Æ
S a
a
§09 nm piimp
Laser transidoflS
13 (un
, —
1.06
> _ .
946 nm
«2 11502 cm->
Rj IWU
[image:21.612.173.445.252.547.2]6725 5765 4500 3922 2S26 2473 2146 2111 is ; S48 311 197 134 0
Figure 1.1. Energy Level Diagram of NdiYAG
Chapter 1
transitions is relatively straight forward resulting in red (750nm), green (532nm) and blue (473nm) light. As well as providing useful source for high density optical data storage*^ there is the possibility of producing an all solid state white light laser source for use in high definition colour projection.
Rapid progress is continuing to be made on all fronts in the field of holosteric lasers. The diode laser pump sources are constantly being improved to give ever higher output powers in better quality beams and over an increasing range of emission wavelengths**»*®. New nonlinear materials are being developed and existing material growth techniques are being improved opening up more opportunities in nonlinear frequency conversion. New solid state gain materials are also being developed for use in diode pumped lasers such as the self-frequency doubling Neodymium Yttrium Aluminium Borate (NYAB) laser®® or widely tunable vibronic gain media such as Cr:Alexandrite^^, CriLiSrAlFô CriLiCaAlFg and CriLiSiGaFg ^4 This wavelength diversity coupled with the exuaordinary frequency stability possible with holosteric lasers means that widely tunable coherent optical sources are now a practical reality.
1.2 Thesis Outline
Chapter 2 contains a brief overview of the remarkable advance made in diode laser technology starting from the basic single emitter device through to the veiy high power one and two dimensional arrays and fibre coupled devices available today. The particular fibre coupled diode laser array used to end pump the NdiYAG lasers in this thesis is also described and a full characterisation of its optical properties is provided.
Chapter 1
to optical slope efficiency measurements were made and the transverse and longitudinal mode behaviour of the NdiYAG laser output was also investigated. A simple rate equation based numerical model of an end-pumped solid laser was developed and used to study the effect of altering various design parameters, such as NdiYAG rod length and output coupling, on the performance of the solid state laser. A comparison between the laser behaviour predicted by this model and the experimental data obtained from the system is also presented.
The next step in the refinement of the basic NdiYAG laser source was to improve its longitudinal mode characteristics from multimode to single mode operation. The basic causes of multi-longitudinal mode operation in lasers are discussed and various methods of mode control are reviewed in Chapter 4. Mode selection by intracavity étalon and by twisted mode were deemed the most appropriate techniques to apply to the open cavity NdiYAG laser described in the previous chapter. These two mode selection techniques were investigated experimentally using the diode end-pumped NdiYAG laser and the relative merits and difficulties encountered with these two systems were described.
In Chapter 5 the numerous environmental and fundamental factors which influence the free-running frequency stability of a solid state laser are considered. This section also charts the gradual improvement in the passive, relative frequency stability of two cw end- pumped single mode holosteric lasers through modifications in cavity design and acoustic isolation. The emphasis here was on reducing the short term laser linewidth in order to ease the demands on bandwidth and gain of any electronic servo loop to be used for further active frequency stabilisation.
Chapter 1
insight into the operation of this system. This is followed by a more rigorous mathematical description. The chapter concludes with a discussion of the ultimate limit of active stabilisation in reducing laser linewidtli.
Following on from this theoretical analysis of active frequency stabilisation. Chapter 7 describes the experimental implementation of the Pound-Drever frequency stabiliser of the diode pumped NdiYAG laser. Include here was design, construction and characterisation of the ultra high finesse (F -12,000) optical reference cavity together with details of the wide bandwidth piezoceramic transducer used to modify the laser frequency. The level of frequency stabilisation achieved with this system was assessed and the limitations of the particular servo loop design used were identified.
One area where frequency stabilised CW lasers are used is in axial mode selection (injection seeding) and frequency control (injection locking) of higher power pulsed and CW laser oscillators by injected signal. Chapter 8 describes experimental work in the use of the narrow linewidth, CW, end-pumped holosteric laser as a "master oscillator" for injection seeding a Q-switched transversely pumped holosteric laser and some preliminary studies of its use for injection locking a second CW, end pumped holosteric laser. The novel use of resonant optical feedback locking as a method for frequency stabilising diode laser pumped NdiYAG lasers is also proposed. Finally, in chapter 9, the accomplishments of the current work are summarised.
1.3 References
1 Yamamoto Y., Kimura T., “Coherent Optical Fibre Transmission System”, IEEE
J. Quant. Elec. QE-17(6) 919 (1981)
Chapter 1
3 Lucente M., Kintzer E. S., Alexander S. B., Fujimoto J. G., Chan V. W. S., “Coherent optical communication with injection-locked high-power semiconductor laser array”. Electron. Lett., 25(17), 1112 (1989)
4 Wallmeroth K., Wandernoth B., Franz J., Meier H., Schorp B., “Towards a Coherent Optical Free-Space Communication System”, Electron. Lett. 26 (9), 572 (1990)
® Heilmann R., Kuschel J., “Absolute frequency Locking of Diode Pumped NdiYAG Laser For Application in Free-Space Optical Communication”, Elec. Lett. 29(9) 810 (1993)
® Cheng E., “Diode-Pumped Lasers Can Communicate in Space”, Laser Focus World 27(7) 99 (1991)
Byer R. L., Gustafson E. K., Trebino R., “Tunable Solid State Lasers For Remote Sensing”, Springer Verlag (1985)
* Hess R. V., Brockman P., Bair C. H„ Barnes J. C., Byuik C. E., Buoncristiani A. M., Magee C. J., “Developement in Tunable Solid State Lasers With High Spectral Purity, High Efficiency and Long Lifetime For Differential Absorption LIDAR”, SPIE Laser Radar Technology And Applications 663 14 (1986)
® Kane T. J., Kozlovsky W. J., Byer R. L., “Coherent Laser Radar at 1.06pm Using NdiYAG Lasers”, Opt. Lett. 12(4) 239 (1987)
Lutz H., Armandillo E., “Laser-Based Remote Sensing From Space”, ESA Bulletin no.66 pps73-79 May (1991)
Chapter 1
12 Weber J., ‘The Detection of Gravitational Waves”, Sci. Am. 224 22 (1971)
Davies P. C. W., “The Search For Gravity Waves”, Cambridge University Press (1980)
14 Kerr G., “Experimental Developments Towards a Long-Baseline Laser Interferometric Gravitational Radiation Detector”, Ph.D, Thesis, University of Glasgow (1986)
1® Letokhov V. S., Moore C. B., “Laser Isotope Separation (Review)”, Sov. J. Quant. Elec. 6(2) 129 (1976)
1® Letokhov V. S., Moore C. B., “Laser Isotope Sepaiation (Review) II”, Sov. J. Quant. Elec. 6(3) 259 (1976)
12 Moore C. B., “The Application of Lasers to Isotope Separation”, Acc. Chem. Res., 6 323 (1973)
1* Zare R. N., “Laser Separation of Isotopes”, Sci. Am. 236(2) 86 (1977)
1® Bollinger J. J., Prestage J. D., Itano W. M., Wineland D. J., “Laser-Cooled-Atomic Frequency Standard”, Phys. Rev. Lett. 54(10) 1000 (1985)
2® Thompson R. C., Barwood G. P., Gill P., “Progress Towards an Optical Frequency Standard Based on Ion Traps”, Appl. Phys. B 46 87 (1988)
21 Barwood G. P., Bell A. S., Gill P., Klein H. A., “Trapped Yb+ as a Potential Optical Frequency Standard”, 4th Symposium on Frequency Standards and Metrology, Ancona, Italy (1988)
Chapter 1
23 Prestage J. D., Tjoelker R. L., Dick G. J., Maleki L., “Ultra-Stable Hg+ Trapped Ion Frequency Standard”, J. Mod. Opt. 39(2) 221 (1992)
24 Arie A., Gustafson E. K., Byer R. L., “Frequency Stabilised Diode Laser Pumped Solid State Lasers: Optical Clocks of the Future”, Optics and Photonics News pps 43-44 December (1992)
25 Prodan J., Migdall A., Phillips W, D., So I., Metcalf H„ Dalibard J., “Stopping Atoms With Laser Light”, Phys. Rev. Lett. 54(10) 992 (1985)
26 Phillips W. D., Metcalf H., “Cooling And Trapping Atoms”, Sci. Am, 256(3) 36 (1987)
27 Wineland D. J., “Laser Cooling”, Physics Today pps 34-40 June (1987)
28 Cohen-Tannoudji C. N., Phillips W. D., “New Mechanisms For Laser Cooling”, Physics Today pps 33-40 October (1990)
29 Zhou B., Kane T. J., Dixon G. J., Byer R. L., "Efficient, Frequency Stable Laser Diode Pumped NdrYAG Laser", Opt. Lett. 10(2) 62 (1985)
30 Kane T. J., Nilsson A. C., Byer R. L., "Frequency Stability and Offset Locking of a Laser Diode Pumped Nd:YAG Monolithic Nonplanar Ring Oscillator", Opt. Lett. 12(3) 175 (1987)
31 Kane T. J., Kozlovsky W. J., Byer R. L„ "Coherent Laser Radar at 1.06|Xm Using Nd:YAG Lasers", Opt. Lett. 12(4) 239 (1987)
Chapter 1
33 Nachman P., Munch J., Yee R., "Diode Pumped, Frequency Stable, Tunable, Continuous Wave NdrGlass Laser", IEEE J. Quant. Elect. 26(2) 317 (1990)
34 Drever R. W. P., Hall J. L., Kowalski F. V., Hough J., Ford G. M., Munley A. L, Ward H., "Laser Phase and Frequency Stabilization Using an Optical Resonator", Appl. Phys. B 31 97 (1983)
35 Shoemaker D., Brillet A., Nary Man C., Crégut O., Kerr G., "Frequency Stabilised Laser Diode Pumped NdiYAG Laser", Opt. Lett. 14(12) 609 (1989)
36 Day T., Gustafson E. K., Byer R. L., "Sub-Hertz Relative Frequency Stabilisation of Two Diode Laser Pumped NdiYAG Lasers Locked to a Fabry- Perot Interferometer", IEEE J. Quant. Elec. 28(4) 1106 (1992)
37 Uehara N., Ueda K-I., “193mHz Beat Linewidth of Frequency Stabilised Laser Diode Pumped NdiYAG Ring Lasers”, Opt. Lett. 18(7) 505 (1993)
38 Peng K C, Wu L A, Kimble H J, “Frequency-stabilised NdiYAG laser with high output power”, Appl. Opt. 24 (7), 938 (1985)
39 Sun Y L, Byer R L, “Submegahertz frequency-stabilized NdiYAG oscillator”. Opt. Lett. 7(9), 408 (1982)
49 Risk W. p., Lenth W., “Room-Temperature, Continuous-Wave, 946nm NdiYAG Laser Pumped By Laser-Diode Arrays And Intiacavity Frequency Doubled To 475nm”, Opt. Lett., 12(12) 993 (1988)
41 Burnham R., Hays A. D., “High Power Diode Array Pumped Frequency Doubled CW NdiYAG Laser”, O pt Lett. 14(1) 27 (1989)
Chapter 1
43 Marshall L. R., Hays A. D., Kaz A., Burnham R. L., “Intracavity Doubled Mode-Locked And CW Diode Pumped Lasers”, IEEE J. Quant. Elec. 28(4) 1158 (1992)
44 Risk W. P., Baumert C. J., Bjorkland G. C., Schellenberg F. M., Lenth W., “Generation Of Blue Light By Intracavity Frequency Mixing of The Laser And Pump Radiation of a Miniature Neodymium: Yttrium Aluminium Garnet Laser”, Appl. Phys. Lett. 52(2) 85 (1988)
45 Malcolm G. P. A., Ebrahimzadeh M., Ferguson A. I., “Efficient Frequency Conversion of Mode-Locked Diode-Pumped Lasers and Tunable All-Solid-State Laser Sources”, IEEE J. Quant. Elec. 28(4) 1172 (1992)
46 Cui Y., Dunn M. H„ Noriie C. J., Sibbett W., Sinclair B. D., Tang Y., Terry J. A. C., “All-Solid-State Optical Parametric Oscillator For The Visible”, Opt. Lett. 17(9) 646 (1992)
47 Cui Y., Withers D. E., Rae C. F., Norrie C. J., Tang Y., Sinclair B, D., Sibbett W., Dunn M. H., “Widely Tunable All-Solid-State Optical Parametric Oscillator For The Visible And Near Infrared”, Opt. Lett. 18(2) 122 (1993)
48 Newman R., “Excitation of Nd Fluorecence in CaWO^ by Recombination Radiation in GaAs”, J. Appl. Phys. 34(2) 437 (1963)
49 Keyes R. J., Quist T. M., “Injection Luminescent Pumping of CaF2:U^‘‘‘ With GaAs Diode Lasers”, AppLPhys. Lett. 4 50 (1964)
Chapter 1
51 Ross M., “YAG Laser Operation by Semiconductor Laser Pumping”, Proc. IEEE 56 196 (1968)
52 Allen R. B., Scalise S. J., “Continuous Operation of a YAlG:Nd Laser by Injection Luminescent Pumping”, Appl. Phys. Lett., 14 188 (1969)
53 Danielmeyer H. G., Ostermayer F. W., “Diode-Pump-Modulated NdiYAG Laser”, J. Appl. Phys. 43(6) 2911 (1972)
54 Rosenkrantz I. J., “GaAs Diode-Pumped NdiYAG Laser”, J. Appl. Phys. 43(11) 4603 (1972)
65 Chinn S. R., Hong H. Y-P., Pierce J. W., “Spiking Oscillations in Diode- Pumped NdPsOi4 and NdAlg(8 0 3 ) 4 Lasers”, IEEE J. Quant. Elec. QE-12(3)
189 (1976)
6 6 Ostermayer F. W., “LED End-Pumped NdiYAG Lasers”, IEEE J. Quant. Elec.
QE-13(1) 1 (1977)
57 Zhou B., Kane T. J., Dixon G. J., Byer R. L., “Efficient, Frequency-Stable Laser Diode Pumped NdiYAG Laser”, Opt. Lett. 10(2) 62 (1985)
6 8 Byer R. L., “Diode Laser-Pumped Solid State Lasers”, Science 239(2) 742 (1988)
69 Fan T. Y., Byer R. L., “Diode Laser-Pumped Solid State Lasers”, IEEE J. Quant Elec. 24(6) 895 (1988)
Chapter 1
61 Special Issue on Diode-Pumped Solid-State Lasers, IEEE J. Quant. Elec. 28(4) 940 (1992)
62 Fields R. A., Bimbaum M., Fincher C. L., “Highly Efficient Nd:YV0 4 Diode- Laser End-Pumped Laser^’, Appl. Phys. Lett. 51 1885 (1987)
63 Tidwell S. C., Seamans J. F., Bowers M. S., “Highly Efficient 60-W TEMqocw
Diode-End-Pumped NdiYAG Laserf, Opt. Lett. 18(2) 116 (1993)
64 Golla D., Freitag I., Zellmer H., Schone W., Kropke I., Welling H., “15W Single-Frequency Operation of a CW, Diode Laser-Pumped NdiYAG Ring Laser”, Opt. Comm. 98(1,2,3) 8 6 (1993)
65 Kasinski J. J., Hughes W., DiBiase D., Bournes P., Burnham R., “One Joule Output From a Diode-Airay-Pumped NdiYAG Laser with Side-Pumped Rod Geometry”, IEEE J. Quant. Elec. 28(4) 977 (1992)
6 6 Holder L. E., Kennedy C., Long L., “One Joule Per Q-Switched Pulse Diode-
Pumped Laser”, IEEE J. Quant. Elec. 28(4) 986 (1992)
67 Malcolm G. P. A., Ferguson A. I., “Mode-Locking of Diode Laser-Pumped Solid-State Lasers”, Opt. Quant. Elec. 24 705 (1992)
68 Baer T. M., Head D. F., Gooding P., Kintz G. J., Hutchison S., “Performance
of Diode Pumped NdiYAG and NdiYLF Lasers in a Tightly Folded Resonator Configuration”, IEEE J. Quant. Elec. 28(4) 1131 (1992)
Chapter 1
79 Leger J. R., Goltsos W. C., “Geometrical Transformation of Linear Diode-Laser AiTays for Longitudinal Pumping of Solid-State Lasers”, IEEE J. Quant. Elec. 28(4) 1088 (1992)
71 Berger J., Welch D. P., Streifer W., Scifres D. R., Hoffman N. J„ Smith J. J., Radecki D., “Fibre-Bundle Coupled, Diode End-Pumped NdiYAG Laser”, Opt. Lett. 13(4) 306 (1988)
72 Howerton P. H., Cordray D, M., “Diode-Pumped Amplifier/Laser Using Leaky Wave Fibre Coupling i An Evalution”, IEEE J. Quant. Elec. 28(4) 1081 (1992)
73 Kane T. J., Byer R. L., " Monolithic, Unidirectional Single-Mode NdiYAG Laser", Opt. Lett. 10(2) 65 (1985)
74 Trutna W. R., Donald D. K., Nazarathy M., "Unidirectional Diode Laser Pumped NdiYAG Ring Laser with a Small Magnetic Field", Opt. Lett. 12(4) 248 (1987)
75 Nilsson A. C., Gustafson E. K., Byer R. L., "Eigenpolarization Theory of Monolithic Nonplanar Ring Oscillators", IEEE J. Quant. Elec. 25(4) 767 (1989)
"^6 Cheng E. A. P., Kane.T. J., "High Power Single Mode Diode Pumped NdiYAG
Laser Using a Monolithic Nonplanar Ring Resonator"' Opt. Lett. 16(7) 478 (1991)
77 Kane T. J., Nilsson A. C., Byer R. L., "Frequency Stability and Offset Locking of a Laser Diode Pumped NdiYAG Monolithic Nonplanar Ring Oscillator", Opt. Lett. 12(3) 175 (1987)
Chapter 1
'^9 Series 200, 500mW output power diode pumped ring laser - Lightwave Electronics, California, USA (1990).
89 Kane T. J., Cheng E. A. P., "Fast Frequency Tuning and Phase Locking of Diode Pumped NdiYAG Ring Laser", Opt. Lett. 13(11) 970 (1988)
81 Trutna W. R., Donald D. K., "Two-Piece Piezoelectrically Tuned Single Mode NdiYAG Ring Laser", Opt. Lett. 15(7) 369 (1990)
82 Day T., Nilsson A. C., Fejer M. M., Farinas A. D., Gustafson E. K., Nabors C. D., Byer R. L., "30Hz Linewidth, Diode Laser Pumped, NdiGGG Nonplanar Ring Oscillators by Active Stabilization", Elec. Lett. 25(13) 810 (1989)
83 Geusic J. K„ Marcos H. M., Von Utert L. G., “Laser Oscillations in Nd-Doped Yttrium Aluminium, Yttiium Gallium and Gadolinium Garnets”, Appl. Phys. Lett. 4(10) 182 (1964)
84 Basu S., Byer R. L., “Average Power Limits of Diode-Laser-Pumped Solid State Lasers”, Appl. Opt. 29(12) 1765 (1990)
85 Koechner W., “Solid State Laser Engineering”, Springer Verlag, 2nd Eddition (1976)
8 6 Zverev G. M., Golyaev Y. D., Shalaev E. A., Shokin A. A., “Neodymium Activated Yttrium-Aluminium Garnet (YAGiNd) Lasers”, J. Sov. Las. Res. 8(3) 189 (1987)
87 Oka M., Kubota S., “Second-Harmonic Generation Green Laser for Higher- Density Optical Disks”, Jpn. J. Appl. Phys. 31(pt.l, no.2B) 513 (1992)
Chapter 1
89 Endriz J. G., Vakili M„ Browder G. S., De Vito M., Haden J. M., Hamagel G. L., Plano W. E., Sakamoto M., Welch D. F„ Willing S., Worland D. P., Yao H. C., “High Power Diode Laser Aii'ays”, IEEE J. Quant. Elec. 28(4) 952 (1992)
99 Hemmati H., “Diode Pumped Self-Frequency Doubled Neodymium Yttrium Aluminium Borate (NYAB) Laser”, IEEE J. Quant Elec. 28(4) 1169 (1992)
91 Scheps R., Gately B., Mayers J. F., Krasinski J. S., Heller D. F., "Alexandrite Laser Pumped by Semiconductor Lasers", Appl. Phys. Lett. 56(23) 2288 (1990)
92 Scheps R., Mayers J. F., Serreze H. B., Rosenberg A., Morris R. C., Long M., "Diode Pumped CriLiSrAlFô Lasers", Opt. Lett. 16(11) 820 (1991)
93 Scheps R., "CriLiCaAlFô Laser Pumped by Visible Laser Diodes", IEEE J. Quant. Elec. 27(8) 1968 (1991)
DIODE
LASER PUMPS FOR SOLID STATE
LASERS
2.1 Introduction
Of key importance to the recent advances in diode laser pumped solid state laser technology has been the remarkable progress made in the development of high power, highly efficient and reliable semiconductor diode lasers. In the field of high power diode lasers research, much of the interest has focused on devices fabricated from ternary alloys of Gai-xAlxAs. This family of laser diodes emit in the near infra-red region of the spectrum with wavelengths ranging from 640-900nm. This wavelength range spans the absorption pump bands of several important laser active ions (e.g. Nd^+, Ho3+, Ei*3+) used in many common solid state laser hosts T2,3,4,5
The highest output powers reported to date for GaAlAs diode lasers have been achieved using integrated linear arrays of diode lasers and hybrid two dimensional stacks of such arrays. For instance, CW power levels over lOOW from a 1cm wide linear array have been reported^. Peak output powers have been pushed even higher by operating diode laser arrays in a long pulse, low duty factor mode to give over 210W peak power from a single 1cm wide array and over 3kW peak power at around 810nm from a 1cm X 1cm two-dimensional stack^.
2.2 Single Emitter Diode Laser Design
Chapter 2 crystalline layers of GaAlAs epitaxially grown on a GaAs substrate. The device derives its name from the fact that the electrical carriers and the optical field generated are confined vertically within the device by different layers of the diode laser structure. At the centre of the device is a two dimensional quantum well active region formed by a very thin (typically lOnm) layer of undoped GaAlAs sandwiched between thicker (~40nm) quantum confinement layers of larger band gap (higher A1 content) undoped GaAlAs. Above and below these layers foiming the quantum well are the p and n-doped layers of GaAlAs. A final capping layer of heavily doped p-type GaAs is deposited on the top of the semiconductor stack to enable ohmic contacts to be formed on the device.
4- 9
PROTON IMPLANT OPTICAL. OUTPUT
V
CLEAVED MIRROR "
METALLIC CONTACT INSULATING LAYER GaAs CAP
CLADDING LAYER ACTIVE LAYER CLADDING LAYER SUBSTRATE (GaAs) METALLIC CONTACT
Figure 2,1. Internal structure of a single quantum-well GaAlAs diode laser.
As constructed, there is no structure in the device to give lateral confinement of electrical carriers or the optical field. Instead, the width of the active region in this dimension is defined by restricting current flow to a narrow channel through the structure. Channelling of the drive current is achieved by leaving only a narrow, well defined strip of high electrical conductivity material on the surface contacting layer and rendering the remainder of this layer electrically insulating by proton bombardment.
Chapter! injected into the active region from the n and p-doped semiconductor layers respectively. These injected carriers become tiapped in the quantum well layer by potential barriers (heterobarriers) at the layer boundaries thus establishing the "population inversion" required for laser action.
Photons generated by the subsequent radiative recombination of these holes and electrons experience strong vertical confinement within the device due to variations in the refractive index of the different semiconductor layers. The optical slab-waveguide formed by these layers has an effective emitting aperture of typically 300-500nm in height. By comparison, optical confinement in the plane of the device is much weaker. In this dimension the width of the optical mode is constrained by the high optical loss suffered in the regions of unpumped semiconductor bounding the active stripe. The width of this so called "gain guided" optical mode is of the order of 5p.m for narrow
stripe devices. Because of the large difference in optical confinement in the planes parallel to and perpendicular to the diode junction the optical mode emitted from the diode laser is highly astigmatic but can nevertheless be focused to a diffraction limited spot by suitable cylindrical optics.
Chapter!
2.2.1 W
avelength Output
The wavelength of the radiation emitted from a diode laser is determined by the bandgap energy of the active region. For GaAlAs devices the laser emission lies in the near infrared part of the spectrum vai-ying from ~640 to ~900nm depending on the exact composition of the device. Whilst coarse wavelength tuning of the diode laser is achieved during the construction stage by choosing the appropriate Aluminium content, fine wavelength adjustment of the finished device is achieved by varying its operating temperature. The emission wavelength shifts at a rate of ~0.3nm/°C due to temperature related changes in the bandgap energy^. The operating temperature of the diode laser can be conveniently and precisely controlled by a thermoelectric (Peltier) device enabling the wavelength of the diode laser output to be matched to the absorption bands of many solid state laser materials resulting in highly efficient pumping.
2.2.2 Maximum Output Power
The maximum optical power available from a diode laser is generally limited by the emission area of the crystal facet and in the case of single, narrow active stripe devices, CW powers as high as 425mW from a 3.1|iim wide active region have been demonstrated^. Under such operating conditions the optical power density at the diode laser facets is extremely high (of the order of a few megawatts/cm^) and operating lifetimes are short. For GaAlAs devices, if the optical power density at the facet exceeds -6MW/cm^, enough localised heating occurs due to non-radiative recombination of
carriers that the mirror facets melt in a catastrophic thermal runaway process^®. To obtain a reasonably long working lifetime >1 0^ hours it is necessary to operate diode
Chapter! To reach higher output powers from diode lasers it is therefore necessary to increase the effective facet area to keep the optical power density within safe limits. In practice, however, it is found that increasing the emitter width of a single stripe to over ~1 0)im
can lead to laser oscillation on many incoherent, unstable filaments^ bz. This filamentary lasing results from an interaction between the optical field and the injected carriers in the active region giving rise to self-focusing of the lasing mode. In a region of high optical field strength within the active stripe, there is correspondingly stronger stimulated recombination of carriers. Associated with this reduced carrier density is an increase in the refractive index in that region and so the lasing filament produces its own optical waveguide. This situation is, however, potentially unstable since there are neighbouring regions of high, undepleted gain. If these high gain regions are wide enough they may be able to support lasing of other filaments. Since such filaments lase essentially independently, their outputs are uncorrelated giving rise to a far-field beam divergence far greater than the diffraction limit. Such uncontrolled filamentary lasing is also potentially hazardous as localised regions of high power density can occur resulting in premature failure of the device. Despite the problems associated with these devices, very high powers have been achieved from single broad area diode lasers. For instance, a 600|im wide stripe GaAlAs broad area laser has been operated up to 5W CW output power^^ and an InGaAsP/GaAs 100pm wide stripe device displayed 5.3W CW output^^.
2.3 High Power Diode Laser Arrays
2.3.1 Diode Laser Arrays and Bars
Chapter 2 techniques and advances in photolithographic processes. These sophisticated epitaxial growth systems aie capable of the very fine compositional control necessary to grow the multilayer diode laser structure and are also able to produce relatively large areas of semiconductor of high uniformity.
PROTON IMPLANT
n -GaAsSUBSTRATE
INDIVIDUAL EMITTING SPOTS
Figure 2.2. Schematic of a high power laser diode airay.
Chapter 2 The maximum output power characteristics of these arrays scale approximately linearly with the number of emitters in the array up to total array widths of the order of 400p.m. However, as the width of the array becomes significantly greater than the laser cavity length, Amplified Spontaneous Emission (ASE) and even transverse lasing can occur across the array causing instability and reduced output power in the desired lasing mode. Such parasitic optical interactions across the array can be suppressed by etching grooves periodically along the length of the device^'^. In this way, very large arrays, known as diode laser bars, have been fabricated. At present practical processing considerations limit laser bar widths to a maximum of 1cm. Such a 1cm wide bar, with a total active aperture size of 7200|im, has been operated up to a 122W CW before failing catastrophically^.
Although the maximum output power for this device was limited by catastrophic melting of the diode laser facet, under CW operating conditions it is more usual for the maximum output power to be limited by heat dissipation in the diode junction. Excessive heat build up in the diode laser manifests itself as a reversible roll-off in output power with increasing drive current^^»'^. By operating the array in a long pulse (~250p,s), low repetition rate (~100Hz) mode known as quasi-CW operation, over heating of the diode junction can be avoided and the peak output power is limited only by the catastrophic failure level.
2.3.2 T
wo Dimensional Diode Laser Stacks
Chapter! when operating the array quasi-CW. To maintain high brightness output from the stack, bars are mounted close together and so waste heat can usually only be removed from the rear surface. Various stack architectures have been developed to improve heat dissipation including the use of impingement coolers^ ^ or synthetic diamond heat spreaders mounted on silicon microchannel coolers^^.
2.3.3 F
ibre Coupled Diode Laser Arrays
An alternative method of multiplexing diode laser arrays to achieve high powers is to couple the output from individual diode laser arrays or bars into multimode optical fibres and form these fibres into tightly packed bundles^ Although the need for accurate alignment of the optical fibres to the diode laser arrays adds an extra element of complexity to the packaging of the device (and subsequent increase in production costs), fibre coupling offers several advantages over two dimensional stacks for high power operation. For instance, fibre coupling enables:
1. the diode laser arrays to be physically separated easing the problem of waste
heat removal.
2. the arrays to be individually temperature tuned for best spectral matching of pump radiation into absorption bands of solid state laser materials. This relaxes the tolerances on spectral matching of diode laser array emission compared with the high degree of spectral and electrical matching required for two dimensional stacks.
3. the diode laser arrays and their associated hardware (current drive units, temperature controllers, heatsinks, etc) to be located remotely from the solid state laser. This enables complete electrical isolation of the laser head and offers the possibility of a more compact laser head design.
4. scalability to higher powers by simply adding more fibre coupled arrays.
Chapter!
6. circular optical output. Fibre optic coupling can be a useful way of
circularising the highly astigmatic, elliptical output beam of a diode laser array. Linear-to-circular fibre bundles have also been used successfully to combine the individual laser array elements of a multiple array, 1cm wide,
diode laser bar into a circular output format"^
However, fibre coupled aiTays exhibit a lower overall electrical to optical efficiency due to quite high coupling losses incurred because of the mismatch between the elliptical output mode shape of the diode laser array and the guided modes of the optical fibre. The optical brightness (optical power per unit area per unit solid angle) of a fibre coupled array is also significantly less than that obtained directly from the airay because of the larger emitting area of the fibre end. To illustrate this, a standard diode laser array emitting 500mW from a 100p,m x 0.5|xm aperture into an elliptical mode with FWHM divergence of 40° in the major axis and 10° in the minor axis has an optical brightness approaching 5MW/cm"^-sr. in contrast, a commercially available fibre coupled version of the same array exhibits an optical brightness of only 1 IkW/cm-^-sr assuming a typical coupling efficiency of 50% into a 100|im diameter core, 0.3 N.A. optical fibre. Significant improvements in optical brightness can be achieved by using a smaller diameter fibre flared into an elliptical cross-section at one end. The elliptically shaped fibre end, butt coupled to the diode array offers a good match to the array's elliptical output greatly improving coupling efficiency. The optical brightness at the output end of a multimode fibre is given by the expression
optical Brightness = Power
Area of fibre end x Solid angle of fibre emission
7--- Ï7 (2.1)
Chapter! where P is the optical power from fibre, r equals the fibre core radius and (N.A.) is the numerical aperture of the fibre. From equation (2.1) it can be seen that the brightness is inversely proportional to the square of the fibre core radius so it is obviously advantageous to taper the fibre core to a small size.
Using this approach Chang-Hasnain and co-w orkers^ at Spectra Diode Laboratories have demonstrated coupling efficiencies in excess of 80% for a lOOp-m wide diode array coupled into a 140pm x 22pm elliptically ended fibre tapered to a circular exit aperture of 50pm diameter. The maximum output power from the fibre reached 850mW CW corresponding to an optical brightness from the fibre end of 150kW/(cm‘^ sr).
2.4 Other Areas In High Power Diode Laser Research
Although edge emitting GaAlAs phased array technology is currently proving a very successful route to high output power, long lifetime semiconductor diode lasers, many other routes to high output powers from diode laser devices are also being explored.
For instance, diode laser arrays are now being developed which emit an output beam perpendicular to the surface of the device as opposed to the more conventional edge emission. Since these surface emitting lasers are not constrained to emit from an exposed edge of the semiconductor, they can be grown anywhere on the substrate enabling one and two dimensional integrated arrays to be fabricated on a single substrate. The problem of waste heat removal which currently plagues edge emitting, high power linear arrays, bars and two dimensional stacks may be eased in the surface emitting geometry as heat can be removed directly through the whole area of the substrate. At present there are three basic configurations of surface emitting devices; vertical cavity lasers^®*^^, 45° defiectors^^»^»^^ and grating surface emitters^^»^®’^^’^*.
Chapter 2 successfully generated coherent high power emission such as Y-junction coupled arrays^^’^®’^^ injection locked arrays^^'^^, hybrid and monolithically integrated Master Oscillator/Power Amplifiers (mOPA)^'^»35,36,37,38 devices and arrays of resonant
antiguides^^*'^®'^h42,43,44
High power diode laser technology is also being applied to other families of semiconductor lasers to provide high power devices operating at alternative wavelengths. Of particular note has been the development of high power strained layer quantum well InGaAs diode lasers emitting in the 910 - 980nm region of the spectrum"^^'^»"^^ for pumping Erbium doped optical fibre amplifiers in long distance optical communication networks'^'^. Also, high power diode laser arrays operating in the red visible part of the spectrum (660-680nm)^®*'^^ are now available and are of considerable interest as pump sources for new vibronic solid state laser gain media such as Cr:Alexandrite^®, CriLiSrAlFg CriLiCaAlFg ^2 and Cr:LiSrGaF6
The wavelength range accessible from diode lasers is also being extended to longer and shorter wavelengths through the development of new semiconductor alloys and diode lasers architectures. The wavelength range now covered by the semiconductor diode laser family extends from 3O|0,m in the mid infra-red generated from PbSnSe lead salt diode lasers down to 447nm from the new cryogenically cooled ZnSe/ZnMgSSe Multiple Quantum Well devices^^’^^*^^’^^.
Given the large spectral coverage possible from diode lasers and the progress being made in the areas of diode laser technology and solid state laser medium research, in particular tunable vibronic systems, there is tremendous potential for developments in the field of diode laser pumped solid state lasers.
2.5 Characterisation of the Pump Diode Laser
CJiapter2 Structure. This device, manufactured by Spectra Diode Labs, is packaged in a TO-3 style canister and comes complete with an internal thermo-electric cooler (TEC) and a photodiode for optical output power monitoring. Laser emission is via a fused silica multimode, step index optical fibre pigtail of dimensions 100|xm core diameter, 140p,m cladding diameter and effective numerical aperture N.A.=0.3. Close coupling of this fibre to the emitting facet of the laser diode array is specified to provide approximately 50% coupling efficiency resulting in a net maximum optical output power of lOOmW from the fibre end.
2.5.1 F
ibre Pigtail Output.
The multimode optical fibre scrambles the spatial mode pattern of the laser diode array resulting in a spatially incoherent but temporally coherent optical emission. The output from the fibre has a circularly symmetric, approximately flat topped intensity profile emerging as a 20° full width at half maximum (FWHM) diverging light cone as shown in figure 2.3.
I
I
PAR FCLD ANOLE @ td«8>
Figure 2,3. Intensity emission pattern from a fibre coupled laser diode array (diagram from Ref.8).
2.5.2 Laser Diode Drivers.
Chapter! thermo-electric cooler (TEC) control circuitry to enable temperature control of the laser diode over the range -20°C - +40°C to an accuracy of ±1°C.
2.5.3 Output P
ower Vs Current Characteristics.
Figure 2.4 shows the light versus current characteristics for the laser diode taken at diode temperatures of 0°C and 25°C. Below threshold the device behaves as a low efficiency Light Emitting Diode (LED) generating only incoherent spontaneous emission. As lasing threshold current is exceeded however, the optical power emitted from the laser diode array increases in a rapid, linear fashion with increasing input current.
100
Î
o SDL800 Internal Meter Scientech Meter
I
40 -6(Ù 20
-Q
0 100 200 300 400 500
Drive Cunent (mA)
Figure 2,4. Output power as a function of drive current for one of the diode laser arrays. Optical power measurements were made using the laser diode’s internal power meter and with an external Scientech power meter.
It can also be seen from figure 2.4 that there is a dependence of laser threshold cunent on device temperature. This change in threshold current with temperature can be approximated by the exponential function^
(threshold at T2) = (threshold at T1) exp[(T^ - Ti)/Tq] (2.2)
Chapter! structure of the device. For the data in figure 2.4, Tq is approximately 150K. The
increase in threshold current with increasing temperature is due in part to the increased energy spread of the carriers injected in to the active region.
2.5.4 Spect
ral Characteristics.
The spectral characteristics of the laser diode array output were studied using a Monospek 1000, 1 metre monochromator calibrated to the 632.8nm and 1064nra laser transitions of a HeNe and a Nd:YAG laser respectively. The highly divergent laser emission from the fibre pigtail was focused onto the input slit of the monochromator by means of a x 10 microscope objective. Transmission through the monochromator was monitored by a large area silicon photodiode (mounted at the monochromator output slit) coupled to an amplifier and chart recorder.
Mode Structure.
A typical output spectrum from the laser diode array is shown in figure 2.5. The device oscillates simultaneously on several (typically 5-10) longitudinal modes contained within a =2nm envelope. These modes are adjacent longitudinal modes of the Fabry-Perot laser cavity and are separated in wavelength by the laser cavity's free spectral range AX given by (see Ref.8)
12
A X «--- (2.3)
2nL.(1.25)
Chapter!
80 mW
60mW
40 mW
25 mW
10 mW (scale >*2)
809 810 811 812 WAVELENGTH Inml
Chapter 2
As well as oscillating on several longitudinal modes, the laser diode array supports oscillation of lateral modes across the evanescently coupled active stripes. It is the presence of these lateral modes or "supermodes"^* which is responsible for the clearly visible substructure superimposed on the spectrum in figure 2.5.
Mode Structure as a Function of Power.
Figure 2.5 shows emission spectra of the laser array for various optical output powers. As might be expected, the number of oscillating modes tends to increase with increasing output power. The envelope of the diode laser emission spectrum also shifts towards longer wavelengths with increasing drive current probably due to an increasing temperature differential between the laser diode chip and the temperature controlled heatsink.
Temperature Tuning.
The tunability of the laser diode output wavelength was investigated. Spectra of the laser output were recorded for various laser diode temperatures in the range 0°C - 30°C whilst maintaining a constant laser output power of 50mW. The primary tuning characteristic indicated by this data (see figure 2.6) is that the 2nm wide wavelength envelope of the
Chapter 2
812
S
I
I
I
808Q 0
1
I 806
I
0.06nm/®C
804
20
10
0
Heatsink Temperature (‘‘C)
(a)
812
Î
1
^
808-I
"t
■g
806-1
a. 804 +
0 10 20 30
Diode Heatsink Temperature (°C) (b)
Chapter!
2.6 References
^ Streifer W., Burnham R. D., Paoli T. L., Scifres D. R., "Phased Array Diode Lasers", Laser Focus 20(6) 100 (1984)
^ Cross P. S., Jacobs R. R., Scifres D. R., "Dynamite Diodes", Photon. Spec. 18(9) 79 (1984)
^ Cross P. S., Hamagel G. L., Streifer W., Scifres D. R., Welch D. P., "Ultra-High Power Semiconductor Diode Laser Arrays", Science 237(9) 1305 (1987)
^ Stiiefer W., Scifres D. R., Hainagel G. L., Welch D, P., Berger J., Sakamoto M., "Advances in Diode Laser Pumps", IEEE J. Quant. Elec. 24(6) 883 (1988)
5 Endriz J. G., Vakili M., Browder G. S., DeVito M., Haden J. M., Hamagel G. L., Plano W. E., Sakamoto M., Welch D. P., Willing S., "High Power Diode Laser Arrays", IEEE J. Quant. Elec. 28(4) 952 (1992)
6 Sakamoto M., Endiiz J. G., Scifres D, R., "120W CW Output Power Prom
Monolithic AlGaAs (800nm) Laser Diode Array Mounted on Diamond Heatsink", Elec. Lett. 28(2) 197 (1992)
Welch D. P., Mehuys D., Parke R., Nam D., Waarts R., Hamagel G., Endriz J., Scifres D ., "High Pow er, T w o-Dimensional L aser A rray", Technical Digest volume 8 from Conference on Lasers and Electro-Optics
(1991), (Optical Society Of America, Washington D.C. 1991) paper CWE5
8 "Laser Diode Operator's Manual and Technical Notes", Spectra Diode Labs, San
Jose, California, (1989)
Chapter!
frequency doubled output (41mW at 428nm)”, IEEE J. Quant. Elect, 27 (6), 1560 (1991)
Heniy C. H., "Catastrophic Damage of AlxGai-xAs Double Heterostructure Laser Material", J. Appl. Phys. 50(5) 3721 (1979)
Thompson G. H. B., "Physics of Semiconductor Laser Devices", John Wiley and Sons LTD., U.K., (1985) ISBN 0-471-27685-5,pp 331-346
12 Yamanaka H., Iwamoto K„ Yamaguchi N., Honda K„ Mamine T., Kojima C., "Progress in Super High Power Laser Diodes With a Broad Area Structure", Technical Digest volume 7 from Conference on Lasers an Electro-Optics (1990),
(Optical Society of America, Washington D.C. 1990) paper CFA2
13 Garbuzov D. Z., Antonishkis N. Y., Bondareu A. D., Gulakov A. B., Zhigulin S. N., Katsavets N. L, Kochergin A. V., Rafailov E. U., "High Power 0.8fim InGaAsP-GaAs SCH SQW Laser", IEEE J. Quant. Elec. 27(6) 1531 (1991)
14 Hamagel G. L., Cross P. S., Scifres D. R., Welch D. P., Lennon C. R., Worland D. P., Bimham R. D., "High-Power Quasi-CW Monolithic Laser Diode Linear Arrays", Appl. Phys. Lett. 49(21) 1418 (1986)
15 Welch D. P., Chan B., Streifer W., Scifres D. R., "High-Power, 8W CW, Single-
Quantum-Well Laser Diode Array", Elec. Lett. 24(2) 113 (1988)
15 Spectra Diode Laboratories technical note "Laser Diode Array Designs for the NOSC High Efficiency Advances Solid State Laser" April 1991.
Chapter!
18 Berger J., Welch D. F., Streifer W„ Scifres D. R., Hoffman N. J., Smith J. J., Rodechi D., "Fibre Coupled, End-Pumped Nd:YAG Laser", Opt. Lett. 13(4) 306 (1988)
19 Chang-Hasnain C., Worland D. P., scifres D. R., "High-Intensity Fibre-Coupled Diode-Laser Array", Elec. Lett 22(2) 65 (1986)
20 Schemr A., Jewell J. L., Lee Y. H., Harbison J. P., Florez L, T., "Fabrication of Microlasers and Microresonator Optical Switches", Appl. Phys. Lett. 55(26) 2724 (1989)
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